1. Field of the Invention
This invention relates generally to a system and method for calibrating a camera on a vehicle and, more particularly, to a system and method for automatically calibrating a camera on a vehicle that tracks points on the ground.
2. Discussion of the Related Art
Modern vehicles sometimes include one or more cameras that provide back-up assistance, take images of the vehicle driver to determining driver drowsiness or attentiveness, provide images of the road as the vehicle is traveling for collision avoidance purposes, provide structure recognition, such as roadway signs, etc. For those applications where graphics are overlaid on the camera images, it is critical to accurately calibrate the position and orientation of the camera with respect to the vehicle. Because of manufacturing tolerances, a separate end-of-line camera calibration, or aftermarket camera adjustment, must be performed on each vehicle for such things as accurate overlay of predicted vehicle path lines.
Some known camera systems do not provide camera calibration, but revert to a default value that may provide a couple of degrees of error. Other camera systems provide a pre-calibration approach where points on the camera image are hand-labeled and feature point locations are hand measured in the vehicle coordinates, such as by providing a checker board pattern of the image. However, these calibration techniques are typically time consuming and must be performed at a service location. Therefore, if the vehicle is traveling and hits a bump or some other obstacle in the road, the camera position could be altered, where the calibration would not be accurate until the vehicle was taken to the service location to be corrected.
Camera calibration involves determining a set of parameters that relate camera image coordinates to vehicle coordinates and vice versa. Some camera parameters, such as camera focal length, optical center, etc., are stable, while other parameters, such as camera orientation and position, are not. For example, the height of the camera depends on the load of the vehicle, which will change from time to time. This change can cause overlaid graphics of vehicle trajectory on the camera image to be inaccurate. Therefore, what is needed is a camera calibration process that automatically calibrates less stable camera parameters as the vehicle is being driven where the vehicle-camera system continually adapts itself over time.